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The principle is pretty easy thanks to the zero dupe, you'll just have to keep your old able2know rewriterule at the end of your new phpBB3 urls, works in all cases (all settings, all url rewriting modes)

You could imagine to keep your current urls as they are, but I think you should start with converting your phpBB2 db to our SEO premod on a local / test server. This way you'll be able to test everything before going online including the redirect and choose which url to keep in the end pretty easily.

When going online, you'd just have to make sure you set the zero duplicate post redirection option to at least "guests" and wait at least two month for the robots.txt implementation

I already created a test account and upgraded it to phpbb3, so I can test various mods on the test server first.

I understand that this mod can create better URLs than able2know did, but my forum already ranks high with the current URLs so I don't want to risk anything. That is why I want to be sure I can keep my current URLs with this mod.

I see that you have several versions of this mod. Which one would be best for my needs?

And you do not risk anything as long as redirects are in place (which is decently easy), many forum already did such migration without an loss in SERPs and SE referrals.
Of course, the PR of the new urls would be 0 at first, but this will only last until next update and again is not related to what really matters : SERPs, indexing and referrals